Electroplating process including pre-wetting and rinsing

ABSTRACT

A plating cell has an inner plating bath container for performing electroplating on a work piece (e.g., a wafer) submerged in a solution contained by the inner plating bath container. A reclaim inlet funnels any solution overflowing the inner plating bath container back into a reservoir container to be circulated back into the inner plating bath container. A waste channel is also provided having an inlet at a different height than the inlet of the reclaim channel. After electroplating, the wafer is lifted to a position and spun. While spinning, the wafer is thoroughly rinse with, for example, ultra pure water. The spin rate and height of the wafer determine whether the water and solution are reclaimed through the reclaim channel or disposed through the waste channel.

This is a division of application Ser. No. 09/096,015, filed Jun. 10,1998.

BACKGROUND OF THE INVENTION

Electroplating is a process for depositing a metal layer on a work piecesuch as a semiconductor wafer. In conventional plating, a wafer isprocessed serially through three separate stages: pre-treatment,plating, and rinse.

After the wafer is prepared for electroplating in one or morepre-treatment cells in the pre-treatment stage, the wafer is transportedto an electroplating cell in the plating stage.

In the electroplating cell, the wafer is immersed in a bath containing asolution of dissolved salts of the metal to be deposited. An anode andcathode terminal are provided in the bath with the wafer attached to thecathode. A direct current is applied between the anode and cathodecausing the dissolved salts to chemically reduce and form thecorresponding metal on portions of the wafer exposed to the current andsolution.

After plating is completed, the wafer is removed from the solution bath.Sometimes, a small amount of rinsing (i.e., pre-rinsing) using ultrapure water is performed in the plating cell causing ultra pure water toenter the solution bath. However, the resulting dilution of thedissolved salt concentration in the bath limits the amount of rinsing inthe plating cell. Therefore, the rinsing over the solution bath isincomplete. In this partially rinsed state, the wafer awaits transportto a separate cell in the rinsing stage for a more thorough rinse anddry.

One problem with this system is that often substantial time (e.g.,minutes) elapses between the electroplating in the plating cell and therinsing while the system awaits availability of a rinse cell, atransport system, or a human operator. Substantial time may elapse when,for example, there is throughput bottleneck in the rinse stage. Duringthis time, solution molecules may stay attached to the wafer due to therelatively low surface energy of the solution. This allows the solution,in some cases, to chemically react with the elements of the wafer,particularly metallic elements. For example, a water-based salt solutiontarnishes a copper work piece if exposed even for a short time. Ascircuit feature dimensions progress farther into the submicron range,such tarnishing can significantly reduce die yields.

Another problem with this system is that the plating cell cannot be usedwhile the wafer awaits transport to the rinsing stage, leaving theplating cell underutilized.

Therefore, there is a clear need for a system that decreases the timelag between the plating and rinse stages.

SUMMARY OF THE INVENTION

A plating cell has a wafer holder that can selectively spin a wafer. Aninner plating bath container is positioned along the axis of rotation ofthe wafer. A first channel (e.g., a reclaim channel) has a first inletat least partially surrounding the axis. A second channel (e.g., a wastechannel) has a second inlet at least partially surrounding the axis ofrotation of the wafer. The second inlet (e.g., the waste inlet) isprovided above the first inlet (e.g., the reclaim inlet). An actuatorselectively moves the wafer holder along the axis so that the waferholder can be positioned at a height below the first inlet.

The following occurs within this single plating cell. A wafer is firstelectroplated by lowering the wafer holder to a position in the innerplating bath container that is below a plating solution level. Afterelectroplating, the wafer is raised out of the plating bath and spun sothat the spun-off water and plating solution enters the reclaim or wasteinlets. When the spun-off water and plating solution enters the wasteinlet, a relatively large volume of rinse solution (e.g., 10 millilitersor more) can be applied (e.g., sprayed) onto the wafer to thoroughlyrinse the wafer. The wafer can also be subsequently dried by spinningthe wafer at a relatively fast spin rate.

The principles of the present invention reduce the time lag between theplating and rinsing stage. Therefore, unwanted chemical reactions (e.g.,corrosion and galvanic reactions) are prevented and a good surfacefinish is achieved. In one embodiment, the wafer surface is completelyand simultaneously covered with water during rinsing. Time delay betweenwater coverage at one point on the wafer surface compared to anotherpoint may result in galvanic corrosion on the wafer surface. When thisoccurs, copper is dissolved where water coverage is adequate but isdeposited where it is inadequate.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic diagram of a plating cell capableof performing both plating and rinsing cell according to one embodimentof the invention.

DETAILED DESCRIPTION OF THE INVENTION

The principles of the present invention solve the problem of time lagbetween the plating and rinse stages by providing a plating cell thatperforms both plating and full rinse. Performing both plating and fullrinse would be undesirable in the prior art plating cell at leastbecause the full rinse would cause rapid dilution of the electroplatingsolution. The plating cell may also have the ability to performpre-wetting.

FIG. 1 is a schematic diagram of a plating cell 100 capable ofperforming both plating and rinsing according to one embodiment of theinvention. Cell 100 includes a pump 122 that pumps solution S fromreservoir container 120, through solution supply channel 116, through aninlet 118 in a base 114 of an inner plating bath container 110, and intoinner plating bath container 110. Solution S is a plating solution thatmay be, for example, a water-based solution with 170 grams per liter ofsulfuric acid and 17 grams per liter of the dissolved plating metal(e.g., copper).

As solution S rises, walls 112 contain solution S until solution Sreaches the lowest portion of a first reclaim inlet 131 at a height “a”(relative to the base 114) in the z-direction parallel to the z-axis.All heights referred to herein are with respect to the z-direction andare referenced to the base 114 of container 110. In FIG. 1, lip 113defines the lower portion of reclaim inlet 131. Reclaim inlet 131 alsohas an upper edge at height “b”.

After the inner plating bath container 110 completely fills withsolution S, excess solution S spills over the lower portion 133 ofreclaim inlet 131 and into a reclaim channel 130. This condition isreferred to herein as “overflow mode.” In overflow mode, walls 112 and132 channel solution S through reclaim channel 130 and into a reclaimdrain 136 provided in a base 134 of channel 130. Reclaim drain 136channels solution S back into reservoir container 120. The condition inwhich solution S follows the above described circulation path isreferred to herein as “circulation mode”. Reclaim inlet 131 maycircumscribe the inner plating bath container 110 about the z-axis.

A second inlet (e.g., a waste inlet) is also provided circumscribedaround the inner plating bath container 110 about the z-axis. Wasteinlet has a lower edge at height “c” and an upper edge at height “d”.Waste solution that enters waste channel 140 is channeled by walls 132and 142 and into a waste drain 146 in a base 144 of waste channel 140.The waste solution is received by a waste system 150 where the solutionis discarded from cell 100.

A wafer holder 190, capable of conveying current, holds a wafer W andcan move in the z-direction (e.g., vertically) by being connected byshaft 180 to an actuator 170. The actuator 170, which may be, forexample, a leadscrew with an encoder, may be controlled by aninstruction on instruction terminal 199 connected to actuator 170.Instruction terminal 199 carries an instruction generated by controller198.

Electroplating may be performed in circulation mode as follows. Inresponse to an instruction on instruction terminal 199, actuator 170positions wafer holder 190 so that wafer W is held in solution S, forexample, at position 1. A current source 192, disposed within the innerplating bath container 110, emits current through solution S and intowafer W. This causes metal to form on the surface of wafer W as themetal salts within the solution S reduce.

Following plating, wafer W is raised to position 2 where wafer holder190 is spun about the z-axis. While wafer holder 190 is spinning, arinse solution (e.g., ultra pure water) is sprayed on the wafer W usingspray nozzle 160, thereby perform partial rinsing (i.e., pre-rinsing).Spray nozzle 160 is positioned so that rinsing solution sprayed by spraynozzle 160 strikes wafer W (e.g., underneath the wafer W). The spin rateat position 2 may range from, for example, 300 to 600 rotations perminute (RPM). At this spin rate, the spun-off solution travelssubstantially horizontal under the influence of gravity. If position 2is such that wafer W is between height “a” and “b” the spun-off solutioncollects within reclaim channel 130 and returns to reservoir container120. To avoid excessively rapid dilution of solution S, the amount ofultra pure water used for rinsing at position 2 is, for example, in therange of from 0 to 8 milliliters.

Following the pre-rinse at position 2, wafer holder is raised toposition 3 for a complete rinse. Once in position 3, the wafer is spunat a rate of, for example, from 300 to 600 RPM while being sprayed withultra pure water. Position 3 is located between height “c” and “d” sothat the solution that is spun-off the wafer W enters waste inlet 141 ofwaste channel 140. Enough ultra pure water is sprayed onto the wafer sothat the wafer W is thoroughly rinsed. A typical range of water volumefor accomplishing this is from 10 to 50 milliliters or from 10 to 150milliliters or even more. The amount of water volume may be very largewithout diluting the solution S because substantially all, if not all,of the water sprayed onto the wafer W is discarded in waste system 150.

A typical elapsed time between the removing of the wafer W from solutionS and the beginning of pre-rinse at position 2 is less than one second.A typical elapsed time for performing pre-rinse is approximately one ortwo seconds. A typical elapsed time between the removal of wafer W fromsolution S and the beginning of the complete rinse at position 3 is lessthan three seconds.

In one embodiment, the wafer surface is completely and simultaneouslycovered with water during rinsing. Time delay between water coverage atone point on the wafer surface compared to another point may result ingalvanic corrosion on the wafer surface. When this occurs, copper isdissolved where water coverage is adequate but is deposited where it isinadequate.

Once the wafer W has been thoroughly rinsed, the wafer W can be spun todry at a spin rate of, for example, from 400 to 800 RPM. Therefore,within a very short period after the completion of electroplating, thewafer W is thoroughly rinsed and dried. Therefore, the wafer W is notexposed to corrosive elements as long as the wafer W might be in theprior art.

The present invention provides several major advantages. First, thepre-rinse stage returns substantially all of the rinse solution backinto the plating solution reducing waste emissions to substantially zeroduring the pre-rinse stage. Second, when spray nozzle 160 is directed atthe wafer W rotating at 30 RPM or more using a nozzle that covers thefull radius of the wafer W, unwanted chemical reactions (e.g., corrosionand galvanic reactions) are prevented. Therefore, a good surface finishis achieved. Third, a volume of rinse solution for the pre-rinse spraymay be selected such that the volume of the circulating solution remainsconstant.

Cell 100 may also be used for pre-wetting by spraying a wetting solution(e.g., ultra pure water) onto the wafer W before electroplating whilethe wafer is positioned over inner plating bath container 110.

Position 2 has been described as being such that wafer W is betweenheight “a” and “b”. However, position 2 may be much higher than height“b” if the spin rate for reclaim is reduced. Furthermore, the lower thespin rate, the higher the wafer holder 190 must be above height “a” inorder for the spun-off solution to enter reclaim inlet 131. The higherthe position above the target inlet 131 or 141, the lower the spin ratemust be to allow the spun-off solution to enter the appropriate inlet131 or 141. Therefore, position 2 may be higher than height “b” andposition 3 may be higher than position “d”. Gravity or other forces willaffect the optimal spin rates at a given position. The above spin ratesassume that the negative z-direction is the direction of the force ofnormal gravity.

Position 2 is described as being the position for pre-rinse in which thespun-off solution is reclaimed. However, wafer holder 190 may skip thepre-rinse stage at position 2 and go directly to the position 3 for thethorough rinse. Alternatively, the pre-rinse stage may be accomplishedat position 3 instead of position 2. This may be accomplish by spinningthe wafer W at a relatively low spin rate of approximately 100 RPM. Inthis case, the spun-off solution is not substantially horizontal, butfalls angularly downward into reclaim inlet 131 or into inner platingbath container 110. After pre-rinse, the spin rate is increased so thatthe spun-off solution enters waste inlet 141.

Wafer holder 190 may be any holder capable of holding a wafer W such asa clamshell apparatus described in a U.S. patent application Ser. No.08/969,984 filed Nov. 13, 1997, entitled “CLAMSHELL APPARATUS FORELECTROCHEMICALLY TREATING SEMICONDUCTOR WAFERS”, now U.S. Pat. No.6,156,167, issued on Dec. 5, 2000 which is incorporated herein byreference in its entirety.

Current source 192 may be, for example, an anode cell as described in aco-pending U.S. patent application Ser. No. 08/969,196 filed Nov. 13,1997, entitled “MEMBRANE PARTITION SYSTEM FOR PLATING OF WAFERS”, nowU.S. Pat. No. 6,126,798, issued on Oct. 3, 2000, which is incorporatedherein by reference in its entirety. Current source 192 may also be acopper sheet connected to base 114 and connected to a voltage source194.

Although inlets 131 and 141 are described as being circumscribedentirely about the z-axis, the circumscription may be only partial toobtain some of the benefit of the present invention.

Although cell 100 has been described as operating in circulation mode inwhich solution S pours over lip 113 and back into reservoir container120, solution S may be at a level below height “a” duringelectroplating. If cell 100 is not in circulation mode, reclaim inlet131 may be higher than waste inlet 141.

Throughout the description and claims, a “channel” is defined as a“structure defining a passage.” Throughout the description and claims,“lower” means “more in the negative z-direction” and “higher” means“more in the positive z-direction.” Thus, a “lowest portion of anobject” means a “portion of an object that is positioned farthest in thenegative z-direction.” In the claims, the recitation of a “first” and“second” item per se should not be interpreted as meaning that the itemsare necessarily different items. For example, a first rinse solution anda second rinse solution may have the same composition or differentcompositions. Furthermore, “first”, “second” and “third” are notnecessarily intended to indicate any time sequencing.

Although the height “c” is described as being above height “b”, height“c” may be the same height as height “b”. Note that the inlet size ofeach inlet 131 and 141 may change by, for example, providing a hinge.

Although the present invention has been described with reference tospecific embodiments, these embodiments are illustrative only and notlimiting. Many other applications and embodiments of the presentinvention will be apparent in light of this disclosure and the followingclaims.

What is claimed is:
 1. A method of rinsing a semiconductor waferfollowing the electroplating of a metal layer on the wafer comprising:lifting the wafer out of an electroplating solution; applying a firstvolume of rinse solution to the wafer; spinning the wafer so as to spinoff the first volume of rinse solution from wafer; and returning atleast a portion of the first volume of rinse solution to theelectroplating solution.
 2. The method of claim 1, further comprising:applying a second volume of a rinse solution to the wafer; spinning thewafer so as to spin off the second volume of rinse solution from thewafer; and discarding at least a portion of the second volume of rinsesolution.
 3. The method of claim 2, wherein applying a second volumecomprises applying over 10 milliliters of rinse solution to the wafer.4. The method of claim 3, wherein applying a second volume comprisesapplying over 10 to 150 milliliters of rinse solution to the wafer. 5.The method of claim 3, wherein applying a second volume comprisesapplying over 50 milliliters of rinse solution to the wafer.
 6. Themethod of claim 2, wherein spinning the wafer so as to spin off thesecond volume of rinse solution comprises spinning the wafer in a rangeof from 300 to 800 rotations per minute.
 7. The method of claim 1,wherein applying a first volume of rinse solution comprises applyingless than 10 milliliters of rinse solution to the wafer.
 8. The methodof claim 7, wherein applying a first volume of rinse solution comprisesapplying 0 to 8 milliliters of rinse solution to the wafer.
 9. Themethod of claim 2 wherein spinning the wafer so as to spin off the firstvolume of rinse solution from the wafer comprises spinning the wafer ata first height above the electroplating solution and wherein spinningthe wafer so as to spin off the second volume of rinse solution from thewafer comprises spinning the wafer at a second height above theelectroplating solution.
 10. The method of claim 9 wherein the firstheight is less than the second height.